The Ohm's Law

Friday, 10 June 2016 11:17:08 Europe/London

One the most important and fundamental laws of electronics is that of ohms law. This law defines the relationships between current, voltage and resistance.

A good way to understand ohms laws is an analogy with a domestic water system - the same way that an electric current flows through a copper wire, water flows through a copper pipe in a water system. Most people don't pay much attention to it when they turn on tap the and water comes out as a result of the pressure that is provided by a pumping station some miles away.

This water pressure is similar to that used to drive electricity around a circuit, the potential difference or pd for short is the term that is used - this is measured in volts. And it's this driving force that is provided by a battery (or in the case of mains electricity a generator at the power station).

What is Ohm's Law?

Ohm's Law is made from 3 mathematical equations that shows the relationship between electric voltage, current and resistance.

Voltage - an analogy would be a huge water tank filled with thousands of gallons of water high on a hill. The difference between the pressure of water in the tank and the water that comes out of the pipe connected at the bottom leading to a faucet is determined by the size of the pipe and the size of the outlet of the faucet. This difference of pressure between the two can be thought of as potential Voltage.

Current - an analogy would be the amount of flow determined by the pressure (voltage) of the water through the pipes leading to a faucet. The term current refers to the quantity, volume or intensity of electrical flow, as opposed to voltage (which refers to the force or "pressure" causing the current flow.

Resistance - an analogy would be the size of the water pipes and the size of the faucet. The larger the pipe and the faucet (less resistance), the more water comes out. The smaller the pipe and faucet (more resistance), the less water comes out. This can be thought of as resistance to the flow of the water current.

All three of these: voltage, current and resistance directly interact in Ohm's law. Change any two of them and you affect the third.

Ohm’s law can be stated in words as “current equals voltage divided by resistance,” and more mathematically as:

I = V / R

Where I is current, V is voltage and R is resistance.

This concurs with the description above, where the voltage provides the raw potential, which is reduced (hence divided by) the resistance to produce the final current. Placing values into the spaces, you could work out the electrical current when you vape at 3.4 V with a 1.8 Ω resistance. This is simply 3.4 divided by 1.8, which comes out to around 1.9 A (after rounding).

Similarly, the equation can be re-arranged if you need to work out something else. You could additionally say that voltage is equal to current multiplied by resistance (V = IR) or that resistance is equal to voltage divided by current (R = V/I).

However, people generally talk about vaping in terms of the wattage, which is a measure of power. This is a measure of energy over time, and the unit of power is the watt (W).

To work out power using the quantities of Ohm’s Law, you can use these equations:

P = V^2 / RP = I^2RP = VI

Power is P in these equations. So if you want to work out how many watts you’re vaping at, you simply work out the voltage squared (at 3.4 V: 3.4^2 = 3.4 × 3.4 = 11.56) and then divide the result by the resistance (at 1.8 Ω, 11.56 / 1.8 = 6.4222… so around 6.4 W). Additionally, you can use the value of the current you’ve worked out previously with either of the remaining two equations (remembering that P = VI means power equals voltage multiplied by current).

The above triangle allows the student to calculate any of the 1 of the 3 variables if the other 2 are known.Cover the variable that you require and perform the resulting calculation.

Consider the circuit shown below:

The resistance of that circuit would be given by R=V/I 24/2 = 12 ohms

The current flowing through the circuit would be given by V/IR 24/12 = 2amps

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